Tailoring the microenvironment of Ti sites in Ti-containing materials for synergizing with Au sites to boost propylene epoxidation

Au sites supported on Ti-containing materials(Au/Ti-containing catalyst)are currently considered as a promising catalyst for the propylene epoxidation owing to the synergistic effect that hydrogen peroxide species formed on Au sites diffuses to the Ti sites to form the Ti-hydroperoxo intermedi-ates and contributes to the formation of propylene oxide(PO).In principle,thermal treatment will significantly affect the chemical and physical structures of Ti-containing materials.Consequently,the synergy between tailored Ti sites with different surface properties and Au sites is highly expected to enhance the catalytic performance for the reaction.Herein,we systematically studied the intrinsic effects of different microenvironments around Ti sites on the PO adsorption/desorption and conversion,and then effectively improved the catalytic performance by tailoring the number of surface hydroxyl groups.The Ti^(Ⅵ) material with fewer hydroxyls stimulates a remarkable enhancement in PO selectivity and H_(2) efficiency compared to the Ti^(Ⅵ) material that possessed more hydroxyls,offering a 7-fold and 4-fold increase,respectively.As expected,the Ti^(Ⅵ+Ⅳ) and Ti^(Ⅳ) materials also exhibit a similar phenomenon to the Ti^(Ⅵ) materials through the same thermal treatment,which strongly supports that the Ti sites microenvironment is an important factor in suppressing PO con-version and enhancing catalytic performance.These insights could provide guidance for the rational preparation and optimization of Ti-containing materials synergizing with Au catalysts for propylene epoxidation.

Styrene epoxidation catalyzed by polyoxometalate/quaternary ammonium phase transfer catalysts: The effect of cation size and catalyst deactivation mechanism

Catalytic epoxidation of alkenes is an important type of organic reaction in chemical industry,and the deep insight into catalyst deactivation will help to develop new epoxidation process.In this work,series of quaternary ammoniums bearing different cationic sizes,i.e.MTOA+(methyltrioctylammonium,[(C_(8)H_(17))_(3)CH_(3)N]+),HTMA+(hexadecyltrimethylammonium,[(C_(16)H_(33))(CH_(3))_(3)N]+) and DMDOA+(dimethyldioctadecylammonium,[(C_(18)H_(37))_(2)(CH_(3))_(2)N]+) were incorporated with polyoxometalate (POM) anions to prepare phase transfer catalysts (PTCs),which were used in the styrene epoxidations.Among them,(MTOA)_(3)PW_(4)O_(24)exhibits the best catalytic performance judged from the highest styrene conversion rate(52%) and styrene oxide selectivity (93%),during which the styrene epoxidation conditions were optimized.Meanwhile,the deactivation mechanism of this kind of PTCs was proposed firstly,i.e.in the case of low H_(2)O_(2) content,the oxidant can only be used in the styrene epoxidation,in which the catalyst can transform into stable Keggin-type POM.But when the content of H_(2)O_(2) is higher,the excess H_(2)O_(2) can reactivate the Keggin-type POM into active (PW_(4)O_(24))_(3)-anions,which can trigger the ring-opening polymerization of styrene oxide.Consequently,the catalyst is deactivated by adhered poly(styrene oxide)irreversibly,which was determined by NMR spectra.In this situation,the active moiety{PO_(4)[WO(O_(2))_(2)]_(4)}_(3)-in phase-transfer catalytic system can break into some unidentified species with low W/P ratio with the presence of epoxides.This work will be beneficial for the design of new PTCs in alkene epoxidation in fine chemical industry.

Study on the epoxidation of olefins with H_(2)O_(2)catalyzed by biquaternary ammonium phosphotungstic acid

Selective epoxidation of olefins is an important field in chemical industry.In this work,we developed a new phosphotungstic acid catalyst{[(C_8H_(17))(CH_(3))_(2)N]_(2)(CH_(2))_(3)}_(1.5){PO_(4)[WO(O_(2))_(2)]_(4)}with long carbon chain and biquaternary ammonium cation.Cyclohexene could be epoxidized to cyclohexene oxide in 96.3%conversion and 98.2%selectivity.The catalyst type,solvent type,catalyst loading,initial molar ratio,temperature,cycle performance and substrate extensibility were studied and optimized,the kinetic parameters about overall reaction and unit reaction were also calculated.Dynamic light scattering analysis was carried out to explain the different catalytic performance between catalysts with different carbon chain length.This novel catalyst and the corresponding dynamics and mechanism study could probably help the industrial application on the epoxidation of cyclohexene with H_(2)O_(2).

One-pot synthesis of ordered mesoporous Cu-KIT-6 and its improved catalytic behavior for the epoxidation of styrene:Effects of the pH value of the initial gel

The heterogeneously copper-catalyzed oxidative cleavage of styrene was studied using copper-doped mesoporous KIT-6（CU-KIT-6_x） prepared via pH adjustment（where x is the pH：1.43,2.27,3.78,3.97,4.24 or 6.62）.Variations in the catalyst structure and morphology with pH values were characterized by X-ray power diffraction,nitrogen adsorption-desorption analysis,transmission electron microscopy and X-ray photoelectron spectroscopy.As the pH value applied during the initial synthesis,the resulting Cu-KIT-6_x exhibited different structural,textural and surface characteristics,especially in terms of specific copper species and copper content At a pH value of 3.78,approximately 4.6 wt%copper（Ⅱ） was successfully incorporated into the framework of the initial KIT-6,in the form of-Cu-O-Si- groups.The catalytic performance of each catalyst was evaluated by following the epoxidation of styrene,employing tert-butyl hydroperoxide as the oxidant and CH_3CN as the solvent.A significant styrene conversion of 43.5%with 86.6%selectivity for the desired styrene epoxide was obtained over the Cu-KIT-63.78.A higher Cu content,an ordered cubic laid mesoporous architecture and various specific textural characteristics all combined to endow the Cu-KIT-63.78 with high catalytic activity and good stability.

Catalytic epoxidation of olefin over supramolecular compounds of molybdenum oxide clusters and a copper complex

The catalytic epoxidation of olefin was investigated on two copper complex-modified molybdenum oxides with a 3D supramolecular structure, [Cu（bipy）]4[Mo15O47].2H2O （1） and [Cu1（bix）][（Cu1bix） （δ-MoVl8O26）0.5] （2） （bipy = 4,4＇-bipyridine, bix = 1,4-bis（imidazole-1-ylmethyl）benzene）. Both compounds were catalytically active and stable for the epoxidation of cyclooctene, 1-octene, and styrene with tert-butyl hydroperoxide （t-BuOOH） as oxidant. The excellent catalytic performance was attributed to the presence of stable coordination bonds between the molybdenum oxide and copper complex, which resulted in the formation of easily accessible Mo species with high electropositivity. In addition, the copper complex also acted as an active site for the activation of t-BuOOH, thus im- proving these copper complex-modified polyoxometalates.

Epoxidation of Unsaturated Fatty Acid Methyl Esters in the Presence of SO_3H-functional Brφnsted Acidic Ionic Liquid as Catalyst

The epoxidation of unsaturated fatty acid methyl esters(FAMEs)by peroxyacetic acid generated in situ from hydrogen peroxide and acetic acid was studied in the presence of SO3H-functional Brnsted acidic ionic liquid (IL)[C3SO3HMIM][HSO4]as catalyst.The effects of hydrogen peroxide/ethylenic unsaturation ratio,acetic acid concentration,IL concentration,recycling of the IL catalyst,and temperature on the conversion to oxirane were studied.The kinetics and thermodynamics of unsaturated FAMEs epoxidation and the kinetics of oxirane cleavage of the epoxidized FAMEs by acetic acid were also studied.The conversion of ethylenic unsaturation group to oxirane, the reaction rate of the conversion to oxirane,and the rate of hydrolysis(oxirane cleavage)were higher by using the IL catalyst.

Structured binder‐free MWW‐type titanosilicate with Si‐rich shell for selective and durable propylene epoxidation

Selective and durable fixed‐bed catalysts are highly desirable for developing eco‐efficient HPPO(hydrogen peroxide propylene oxide)process.The powder titanosilicate catalysts must be shaped before being applied in industrial processes.As the essential additives for preparing formed catalysts,binders are usually the catalytically inert components,but they would cover the surface and pore mouth of zeolite,thereby declining the accessibility of active sites.By recrystallizing the binder(silica)/Ti‐MWW extrudates with the assistance of dual organic structure‐directing agents,the silica binder was converted into MWW zeolite phase to form a structured binder‐free Ti‐MWW zeolite with Si‐rich shell,which enhanced the diffusion efficiency and maintained the mechanical strength.Meanwhile,due to the partial dissolution of Si in the Ti‐MWW matrix,abundant silanol nests formed and part of framework TiO4 species were transferred into open TiO_(6)ones,improving the accumulation and activation ability of H_(2)O_(2)inside the monolith.Successive piperidine treatment and fluoridation of the binder‐free Ti‐MWW further enhanced the H_(2)O_(2)activation and oxygen transfer ability of the active Ti sites,and stabilized the Ti‐OOH intermediate through hydrogen bond formed between the end H in Ti‐OOH and the adjacent Si‐F species,thus achieving a more efficient epoxidation process.Additionally,the side reaction of PO hydrolysis was inhibited because the modification effectively quenched numerous Si‐OH groups.The lifetime of the modified binder‐free Ti‐MWW catalyst was 2400 h with the H_(2)O_(2)conversion and PO selectivity both above 99.5%.

Green and efficient epoxidation of methyl oleate over hierarchical TS-1

The epoxidation of methyl oleate(MO)was conducted in the presence of aqueous H2O2 as the oxidant and hierarchical TS-1(HTS-1)as the catalyst;the catalyst was synthesized using polyquaternium-6 as the mesopore template.The effects of various parameters,i.e.,H2O2/C=C molar ratio,oxidant concentration,amount of the catalyst,reaction temperature,and time,were systematically studied.Furthermore,response surface methodology(RSM)was used to optimize the conditions to maximize the yield of epoxy MO and to evaluate the significance and interplay of the factors affecting the epoxy MO production.The H2O2/C=C molar ratio and catalyst amount were the determining factors for MO epoxidation,wherein the maximum yield of epoxy MO reached 94.9%over HTS-1 under the optimal conditions.

Dioxygen Affinities and Catalytic Epoxidation Performanceof Transition-Metal Hydroxamates

The dioxygen affinities and catalytic epoxidation performance of transition-metal hydroxamates were investigated for the first time. The effects of substituents on these properties were also discussed in the paper.

Epoxidation of Cyclohexene with Oxygen in Ultrasound Airlift Loop Reactor

Epoxidation of cyclohexene to cyclohexene oxide was studied in a new type reactor—the ultrasound airlift loop reactor. The influences of ultrasound intensity, molar ratio of isobutyraldehyde to cyclohexene and oxy-gen gas flow rate on the conversion of cyclohexene and selectivity of cyclohexene oxide were investigated and dis-cussed, and the optimal operation condition was found, under which 95.2% conversion of cyclohexene and 90.7% selectivity of cyclohexene oxide were achieved. The ultrasonic airlift loop reactor utilizes the synergistic effect of sonochemsitry and higher oxygen transfer rate. Possible reaction mechanisms were outlined and the reason of ul-trasound promotion of epoxidation reactionwas analyzed.

Solvothermal synthesis of Co-substituted phosphomolybdate acid encapsulated in the UiO-66 framework for catalytic application in olefin epoxidation

Hybrid composites of phosphomolybdic acid@UiO-66(PMo12@UiO-66)and Co-substituted phosphomolybdic acid@UiO-66(PMo11Co@UiO-66)were synthesized using the direct solvothermal method.A variety of characterization results demonstrated that phosphomolybdic acid(PMo12)or Co-substituted phosphomolybdate acid(PMo11Co)clusters are uniformly dispersed in the cages of Zr-based metal-organic UiO-66 frameworks.The catalytic properties of these hybrid composites were investigated by applying the epoxidation of olefins with tert-butyl hydroperoxide as the oxidant.Compared to PMo12@UiO-66,PMo11Co@UiO-66 showed a much higher catalytic activity and was simply recovered by filtration and reused for at least ten runs without significant loss of catalytic activity.Particularly,PMo11Co@UiO-66 can efficiently convert cyclic olefins like limonenes to epoxides,and its selectivity to 1,2-limonene oxide reached 91%in the presence of a radical inhibitor such as hydroquinone.The excellent catalytic activity and stability of the hybrid composite PMo11Co@UiO-66 are mainly attributed to the uniform distribution of highly active PMo11Co units within the smaller cages of UiO-66,to the suitable surface polarity of the hybrid composite for facilitating the access of reagents and solvent,and to the strong interface-interactions between the polyoxometalate and the UiO-66 framework.

Effects of the Amount of Tetrapropyl Ammonium Hydroxide in Synthesis on TS-1 Properties and Catalytic Performance in Epoxidation of Propylene

Titanium silicalite-1(TS-1) was synthesized by a hydrothermal synthesis method with different amounts of tetrapropyl ammonium hydroxide(TPAOH) as template.The as-prepared TS-1 was characterized by scanning electron microscopy,X-ray powder diffraction,Fourier-transform infrared resonance spectroscopy,ultravioletvisible diffuse reflectance spectroscopy and nitrogen physical adsorption and desorption,and studied in the propylene epoxidation with hydrogen peroxide in a fixed bed reactor.The results showed that the amount of TPAOH had a strong influence on the grain morphology,the amount of framework Ti,and the average particle sizes of TS-1.With the increase of TPAOH amount in the synthesis(the molar ratio of TPAOH/SiO_2 increasing from 0.25 to 0.45),the morphology changed gradually from ellipsoids to cubes,the particle sizes of TS-1 decreased slightly,the amount of the framework Ti increased appreciably,and the catalytic stability in the propylene epoxidation increased markedly.Moreover,all the catalysts had the same selectivity to propylene epoxide.However,when the molar ratio of TPAOH/SiO_2 was further increased to 0.55,the particles became large hexagons with the size distribution in a wide range,and the catalytic stability decreased sharply although the amount of the framework Ti increased further,which can be attributed to the long diffusion paths of the reactants in the zeolite.

Synergetic photo-epoxidation of propylene with molecular oxygen over bimetallic Au–Ag/TS-1 photocatalysts

Au-Ag bimetallic nanoparticle‐supported microporous titanium silicalite‐1catalysts were prepared via a hydrothermal‐immersion method,and their structures were examined.These materials serve as efficient catalysts for the photosynthesis of propylene oxide via the epoxidation of propene.The Au/Ag mass ratio and reaction temperature were demonstrated to have significant effects on the catalytic activity and selectivity of propylene oxide.The optimal formation rate(68.3μmol/g·h)and selectivity(52.3%)toward propylene oxide were achieved with an Au:Ag mass ratio of4:1.Notably,the strong synergistic effect between Au and Ag resulted in superior photocatalysis of the bimetallic systems compared with those of the individual systems.A probable reaction mechanism was proposed based on the theoretical and experimental results.

Direct hydrothermal synthesis of Mo-containing MFI zeolites using Mo-EDTA complex and their catalytic application in cyclohexene epoxidation

A series of Mo-containing MFI zeolites with different Mo loadings(Mo-MFI-n,n represent the initial Si/Mo molar ratio)was hydrothermally synthesized by using tetrapropylammonium hydroxide as the template and Mo-EDTA complex as the Mo source.Various characterization results demonstrated that the use of the Mo-EDTA complex is beneficial for the incorporation of more Mo species into the MFI-type zeolites.The special complexing capability of EDTA^(2–)plays a critical role in adjusting the release rate of the Mo species to combine with the Si tetrahedron species during the zeolite growth process,thus leading to a uniform distribution of Mo in the MFI framework.In addition,a small portion of extra-framework Mo clusters may be distributed inside the channels or near the pore window of the zeolites.The catalytic properties of these Mo-containing MFI zeolites were evaluated for the epoxidation of cyclohexene with H_(2)O_(2)as the oxidant.The composition-optimized catalyst,Mo-MFI-50,efficiently converted cyclohexene to the corresponding epoxide with a relatively high conversion(93%)and epoxide selectivity(82%)at 75℃after 9 h of reaction.Moreover,the resultant Mo-containing MFI catalyst exhibited excellent structural stability and recoverability and was easily recycled by simple filtration without the need for calcination treatment.

Direct gas-phase epoxidation of propylene to propylene oxide using air as oxidant on supported gold catalyst

Gold catalysts supported on SiO2, TiO2, TiO2-SiO2, and ZrO2-SiO2 supports were prepared by impregnating each support with a basic solution of tetrachloroauric acid. X-ray diffraction （XRD）, transmission electron microscopy （TEM）, and X-ray photoelectron spectroscopy （XPS） techniques were used to characterize their structure and surface composition. The results indicated that the size of gold particles could be controlled to below 10 nm by this method of preparation. Washing gold catalysts with water could markedly enhance the dispersion of metallic gold particles on the surface, but it could not completely remove the chloride ions left on the surface. The catalytic performance of direct vapor-phase epoxidation of propylene using air as an oxidant over these catalysts was evaluated at atmospheric pressure. The selectivity to propylene oxide （PO） was found to vary with reaction time on the stream. At the reaction conditions of atmosphere pressure, temperature 325 ℃, feed gas ratio V（C3H6）/V（O2）= 1/2, and GHSV =6000h^-1, 17.9% PO selectivity with 0.9% propylene conversion were obtained at initial 10 min for Au/SiO2 catalyst. After reacting 60 min only 8.9% PO selectivity were detected, but the propylene conversion rises to 1.4% and the main product is transferred to acrolein （72% selectivity）. Washing Au/TiO2-SiO2 and Aa/ZrO2-SiO2 samples with magnesium citrate solution could markedly enhance the activity and PO selectivity because smaller gold particles were obtained.

Engineering three-layer core–shell S-1/TS-1@dendritic-SiO_(2) supported Au catalysts towards improved performance for propene epoxidation with H_(2) and O_(2)

The advocacy of green chemical industry has led to the development of highly efficient catalysts for direct gas-phase propene epoxidation with green,sustainable and simple essence.The S-1/TS-1@dendritic-SiO_(2) material with three-layer core–shell structure was developed and used as the support for Au catalysts,which showed simultaneously fantastic PO formation rate,PO selectivity and stability(over 100 h)for propene epoxidation with H_(2) and O_(2).It is found that silicalite-1(S-1)core and the middle thin layer of TS-1 offer great mass transfer ability,which could be responsible for the excellent stability.The designed dendritic SiO_(2) shell covers part of the acid sites on the external surface of TS-1,inhibiting the side reactions and improving the PO selectivity.Furthermore,three kinds of SiO_(2) shell morphologies(i.e.,dendritic,net,mesoporous shell)were designed,and relationship between shell morphology and catalytic performance was elucidated.The results in this paper harbour tremendous guiding significance for the design of highly efficient epoxidation catalysts.

Propylene Epoxidation by Dioxygen over Catalyst Copper-loaded TiO_2

A copper-TiO2 based catalyst（Cu-OH-Cl-TiO2） was prepared through a slurry impregnation approach and the catalyst was found to be active and selective for the epoxidation of propylene by dioxygen. With a feed gas of 10% C3H6, 10% O2 and 80% N2 at a gas hourly space velocity（GHSV） of 4000 h-1, a propylene conversion of 4.8% and a propylene oxide（PO） selectivity of 38.9% were achieved over the obtained Cu-OH-Cl-TiO2 catalyst at a reaction temperature of 500 K. It revealed that Cu2＋ provided by crystalline Cu2（OH）3Cl plays a key role in catalytic conversion of propylene to PO.

Enhancing the dynamic electron transfer of Au species on wormhole-like TS-1 for boosting propene epoxidation performance with H_(2) and O_(2)

Engineering unique electronic structure of catalyst to boost catalytic performance is of prime scientific and industrial importance.Herein,the identification of intrinsic electronic sensitivity for direct propene epoxidation was first achieved over highly stable Au/wormhole-like TS-1 catalyst.Results show that the electron transfer of Au species can be regulated by manipulating the dynamic evolutions and contents of Au valence states,thus resulting in different catalytic performance in 100 h time-on-stream.By DFT calculations,kinetic analysis and multicharacterizations,it is found that the Au^(0) species with higher electronic population can easily transfer more electrons to activate surface O_(2) compared with Au^(1+) and Au^(3+) species.Moreover,there is a positive correlation between Au^(0) content and activity.Based on this correlation,a facile strategy is further proposed to boost Au^(0) percentage,resulting in the reported highest PO formation rate without adding promoters.This work harbors tremendous guiding significance to the design of highly efficient Au/Ti-containing catalyst for propene epoxidation with H_(2) and O_(2).

Gas-phase epoxidation of propylene by molecular oxygen over Ag-Cu-Cl/BaCO_3 catalyst:Effects of Cu and Cl loadings

Ag‐Cu‐Cl/BaCO3 catalysts with different Cl and Cu loadings, prepared by the reduction deposition impregnation method, were investigated for gas‐phase epoxidation of propylene by molecular oxygen and characterized by X‐ray diffraction, X‐ray photoelectron spectroscopy and O2 temperatureprogrammed desorption. Ag‐Cu‐Cl/BaCO3 catalyst with 0.036 wt% Cu and 0.060 wt% Cl exhibitedthe highest catalytic performance for gas‐phase epoxidation of propylene by molecular oxygen. Apropylene oxide selectivity of 83.7% and propylene conversion of 1.2% were achieved under thereaction conditions of 20% C3H6‐10% O2‐70% N2, 200 °C, 0.1 MPa and 3000 h?1. Increasing the Clloading allowed Ag to ensemble easier, whereas changing the Cu loading showed little effect on Agcrystallite size. The appropriate Cl loading of Ag‐Cu‐Cl/BaCO3 catalyst can reduce the dissociationadsorption of oxygen to atomic oxygen species leading to the combustion of propylene to CO2, whichbenefits epoxidation of propylene by molecular oxygen. Excessive Cl loading of Ag‐Cu‐Cl/BaCO3catalyst decreases propylene conversion and propylene oxide selectivity remarkably because of Clpoisoning. The appropriate Cu loading of Ag‐Cu‐Cl/BaCO3 catalyst is efficient for the epoxidation ofpropylene by molecular oxygen, and an excess Cu loading decreases propylene oxide selectivitybecause the aggregation of Cu species increases the exposed surfaces of Ag nanoparticles, whichwas shown by slight increases in atomic oxygen species adsorbed. The appropriate loadings of Cu and Cl of Ag‐Cu‐Cl/BaCO3 catalyst are important to strike the balance between molecular oxygen and atomic oxygen species to create a favorable epoxidation of propylene by molecular oxygen.

Selective propylene epoxidation in liquid phase using highly dispersed Nb catalysts incorporated in mesoporous silicates

Selective propylene epoxidation to propylene oxide(PO) with hydrogen peroxide(H_2O_2) was carried out in a catalytic semi-batch reactor.High propylene epoxidation activity(44 h^(-1)) was observed over Nb based mesoporous silicate materials Nb-TUD-1 under mild operating conditions.The physical and chemical properties of the Nb based silicates characterized using BET,FTIR,TPD,TEM and UV–Vis revealed that the site isolation and surface acidity are crucial for PO production.Catalyst synthesis methods were investigated for their effects on PO productivity,PO selectivity and H_2O_2 utilization efficiency.It is found that Nb-TUD-1 material synthesized by the sol–gel method is more active and selective than impregnated materials for liquid phase propylene epoxidation.Surface characterization confirms that thus synthesized Nb-TUD-1 catalysts have more Lewis acidity and less Bronsted acidity compared to the catalysts by impregnation.